1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and more particularly relates to an apparatus that folds a sheet bundle and performs bookbinding processing.
2. Description of the Related Art
There is a conventional image forming apparatus (e.g., a copying machine, a laser beam printer, etc.) equipped with a sheet processing apparatus that can appropriately fold respective sheets discharged from the image forming apparatus, or stitch the sheets along their center lines and then fold the stitched sheets for saddle stitch bookbinding.
In the saddle stitch bookbinding processing, if the number of sheets constituting a sheet bundle is large (e.g., 20 or more) and the bundle of sheets is bent for bookbinding, a folded spinal portion of a finished product may have a curvature or bow. A folded sheet bundle finished in this manner may still open somewhat even after the sheet bundle is firmly pressed and folded giving it an unattractive appearance. Such a curvature or bow in the folded spinal portion also makes it more difficult to stack a number of such folded sheet bundles due to the variation in thickness.
To improve the appearance and flatness of a folded sheet bundle, a sheet processing apparatus discussed in U.S. Pat. No. 6,692,208 includes a pressing roller that can travel along a folded spinal portion of the folded sheet bundle to deform or squash a curvature of the folded spinal portion.
FIGS. 18A to 18C illustrate an example configuration of a conventional sheet processing apparatus. When the sheet processing apparatus performs processing for deforming a folded spinal portion of a folded sheet bundle, a pair of belt conveying means 1106 and 1107 conveys a folded sheet bundle S until the folded spinal portion collides against a positioning means 1105 as illustrated in FIG. 18A. After the folded spinal portion abuts against the positioning means 1105, the belt conveying means 1106 and 1107 continuously rotate a predetermined amount to further convey the folded sheet bundle S while causing slip on their surfaces. This is effective to correct a skew of the folded sheet bundle S and accurately adjust the position of the folded sheet bundle S.
Next, as illustrated in FIG. 18B, the folded sheet bundle S is held between a pair of grip means 1102 and 1103 with its folded spinal portion protruding. Namely, the grip means 1102 and 1103 cooperatively fix the folded sheet bundle S at a position adjacent to the folded spinal portion. The positioning means 1105 moves to a retreat position. Then, as illustrated in FIGS. 19A and 19B, a pressing roller 1104 travels in a direction indicated by an arrow while applying pressure, against or opposed to the conveying direction, to the folded spinal portion of the curved sheet bundle S that protrudes from the grip means 1102 and 1103.
Thus, the folded spinal portion of the curved sheet bundle S can be deformed to create a substantially flat surface along the folded spinal portion, the substantially flat surface preferably being substantially perpendicular to the front and rear cover of the sheet bundle. Then, as illustrated in FIG. 18C, the belt conveying means 1106 and 1107 convey and discharge the deformed folded sheet bundle S to a sheet discharge tray 1108.
As discussed in Japanese Patent Application Laid-Open No. 2000-198613, there is a conventional cutting apparatus (i.e., a trimmer) that performs processing for cutting a sheet bundle along one end (i.e., opened end) thereof after the sheet bundle is subjected to saddle stitch bookbinding. A sheet bundle finished by the saddle stitch bookbinding has a central sheet that protrudes at its opened end compared to other sheets (the protrusion being greatest with respect to a sheet adjacent to the front or rear cover) if the thickness of the sheet bundle is large at a bending portion of the folded sheet bundle. Therefore, the opened end portion of the sheet bundle takes a convex shape. A product having a good appearance can be obtained by cutting the convex shape into a flat shape.
FIG. 20 illustrates a conventional cutting apparatus (i.e., a sheet processing apparatus) that includes an upper cutting blade 2101 and a lower cutting blade 2102 that cooperatively cut a folded sheet bundle S that is finished by the saddle stitch bookbinding processing. To cut the folded sheet bundle S, a conveyance belt 2111 conveys the folded sheet bundle S to a position between the upper and lower cutting blades 2101 and 2102 and stops rotating if a folded spinal portion of the folded sheet bundle S collides against a stopper 2127. In ordinary processing for cutting the opened end of a folded sheet bundle in a state where the folded spinal portion of the folded sheet bundle abuts against the positioning means 1105, a cutting position is spaced a predetermined amount from the folded spinal portion. The position of the stopper 2127 is changeable in the conveyance direction according to the size of the folded sheet bundle S and a determined cutting amount.
A gripping unit 2136 firmly holds the folded sheet bundle S during a cutting operation in a state where the folded sheet bundle S abuts against the stopper 2127. Then, the upper cutting blade 2101 moves downward and, after it reaches the lower cutting blade 2102, cuts the folded sheet bundle S. The folded sheet bundle S, being cut in this manner, is conveyed to a bundle storage (not illustrated). The cutting scrap generated in this cutting operation falls into a scrap box 2108 positioned below the cutting blades 2101 and 2102.
In FIG. 20, a swing guide 2114 can guide the folded sheet bundle S from the conveyance belt 2111 to the lower cutting blade 2102. When the folded sheet bundle S is cut, the swing guide 2114 moves (i.e., rotates) downward to let the scrap fall into the scrap box 2108. When the cutting processing is completed, the swing guide 2114 moves upward to guide the next folded sheet bundle.
However, to improve the appearance of both the folded spinal portion and the opened end portion, it is usual to perform the above-described two edge portion processing. As example processing applied to edge portions of a sheet, the above-described deforming processing may be applied to a folded spinal portion and the arranging processing (i.e., cutting processing) may be applied to an opened end portion to improve the quality of the processed folded sheet bundle. In this case, it is desired to cut the opened end of the sheet bundle after completing the deforming processing. More specifically, after finishing the deforming processing applied to the folded spinal portion of the folded sheet bundle S, the deformed folded spinal portion of the sheet bundle S collides against the stopper held at a position determined according to the size or thickness of the sheet bundle S. Then, the cutting processing is performed on the sheet bundle S being thus positioned.
However, the applicant has identified that if a conventional sheet processing apparatus or a conventional image forming apparatus performs deforming processing on a folded spinal portion and arranging processing on an opened end portion, a folded sheet bundle S1 having been subjected to the deforming processing becomes shorter in length, by a deforming amount of the folded spinal portion, in the sheet conveyance direction (i.e., the length from the folded spinal portion to the opened end) as illustrated in FIG. 21.
Therefore, if two folded sheet bundles S1 and S2 to be cut at their opened ends have the same thickness, the length of the folded sheet bundle S1 having been subjected to the deforming processing in the sheet conveyance direction is shorter than the length of the folded sheet bundle S2 not subjected to the deforming processing in sheet conveyance direction by an amount equivalent to a deforming amount L. For example, as illustrated in FIG. 21, “A” represents a length from a folded spinal portion of respective folded sheet bundles S1 and S2 to the upper cutting blade 2101, and “C” represents an edge portion cutting amount (i.e., cutting length) of the folded sheet bundles S1 and S2 having been positioned.
In this case, the folded sheet bundle S1 having been subjected to the deforming processing is shorter in actual cutting length than the folded sheet bundle S2 not subjected to the deforming processing by an amount equivalent to the deforming amount L. When an image forming apparatus performs image formation on each sheet, a distance from one edge portion to an image writing position on the sheet is constant in the sheet conveyance direction of a folded sheet bundle. Therefore, if the cutting processing is performed on a folded sheet bundle including a plurality of sheets on which images are formed, the cutting position at the opened end portion needs to be separated from an image forming portion.
In general, to improve the quality of a folded sheet bundle, it is desired to cut a curved shape of the opened end portion into a flat shape. To this end, as a condition for the cutting processing applied to the folded sheet bundle S2 not subjected to the deforming processing, the above-described image formation may be taken into consideration to determine the distance from the folded spinal portion of the folded sheet bundle S2 to the cutting position. However, if the distance between the folded spinal portion and the cutting position determined for the folded sheet bundle S2 is directly used in the cutting processing for the folded sheet bundle S1 having been subjected to the deforming processing, the cutting position may shift into a region corresponding to the curved shape of the opened end portion, as the length in the sheet conveyance direction is shortened by the deforming amount L. As a result, the opened end portion of the folded sheet bundle S1 may not be cut into a flat shape.